O.S.GABRIELYAN,
I.G. OSTROUMOV,
A.K.AKHLEBININ

START IN CHEMISTRY

7th grade

Continuation. See the beginning in No. 1/2006

§ 2. Observation and experiment as methods
studying science and chemistry

A person gains knowledge about nature using such an important method as observation.

Observation- this is the concentration of attention on cognizable objects with the aim of studying them.

With the help of observation, a person accumulates information about the world around him, systematizes it and searches for patterns in this information. The next important step is to search for reasons that explain the patterns found.

In order for observation to be fruitful, a number of conditions must be met.

1. It is necessary to clearly define the subject of observation, what the observer’s attention will be drawn to - a specific substance, its properties or the transformation of some substances into others, the conditions for the implementation of these transformations, etc.

2. The observer must know why he is conducting the observation, i.e. clearly formulate the purpose of the observation.

3. To achieve your goal, you can draw up an observation plan. And for this it is better to make an assumption about how the observed phenomenon will occur, i.e. put forward hypothesis. Translated from Greek “hypothesis” ( hypo"thesis) means "guess". A hypothesis can also be put forward as a result of observation, i.e. when some result is obtained that needs to be explained.

Scientific observation differs from observation in the everyday sense of the word. As a rule, scientific observation is carried out under strictly controlled conditions, and these conditions can be changed at the request of the observer. Most often, such observation is carried out in a special room - a laboratory (Fig. 6).

Observation that is carried out under strictly controlled conditions is called experiment.

The word "experiment" ( experimentum) is of Latin origin and is translated into Russian as “experience”, “test”. An experiment allows you to confirm or refute a hypothesis that was born from observation. This is how it is formulated conclusion.

Let's conduct a small experiment with which we will study the structure of the flame.

Light a candle and carefully examine the flame. You will notice that it is not uniform in color. The flame has three zones (Fig. 7). Dark Zone 1 located at the bottom of the flame. This is the coldest zone compared to others. The dark zone is surrounded by the brightest part of the flame 2 . The temperature here is higher than in the dark zone, but the highest temperature is in the upper part of the flame 3 .

To make sure that different zones of the flame have different temperatures, you can conduct this experiment. Place a splinter (or match) into the flame so that it crosses all three zones. You will see that the splinter is more charred where it hits the zones 2 And 3 . This means the flame is hotter there.

The question arises: will the flame of an alcohol lamp or dry fuel have the same structure as the flame of a candle? The answer to this question can be two assumptions - hypotheses: 1) the structure of the flame will be the same as the flame of a candle, because it is based on the same combustion process; 2) the structure of the flame will be different, because it arises as a result of the combustion of various substances. In order to confirm or refute this or that hypothesis, let's turn to the experiment - let's conduct an experiment.

Using a match or splinter, we examine the structure of the flame of an alcohol lamp (you will become familiar with the structure of this heating device during practical work) and dry fuel.

Despite the fact that the flames in each case differ in shape, size and even color, they all have the same structure - the same three zones: the inner dark (coldest), the middle luminous (hot) and the outer colorless (hottest).

Consequently, the conclusion from the experiment can be the statement that the structure of any flame is the same. The practical significance of this conclusion is as follows: in order to heat any object in a flame, it must be brought into the hottest place, i.e. to the upper part of the flame.

It is customary to document experiments in a special journal, which is called a laboratory journal. An ordinary notebook is suitable for this, but the entries in it are not quite ordinary. The date of the experiment, its name are noted, and the progress of the experiment is often presented in the form of a table.

Try to describe an experiment to study the structure of a flame in this way.

The great Leonardo da Vinci said that sciences that were not born from experiment, this basis of all knowledge, are useless and full of errors.

All natural sciences are experimental sciences. And to set up an experiment, special equipment is often needed. For example, in biology, optical instruments are widely used that make it possible to enlarge the image of the observed object many times: magnifying glass, magnifying glass, microscope. Physicists use instruments to measure voltage, current, and electrical resistance when studying electrical circuits. Geographers have special instruments - from the simplest (for example, a compass, weather balloons) to unique space orbital stations and research vessels.

Chemists also use special equipment in their research. The simplest of them is, for example, the already familiar heating device, an alcohol lamp, and various chemical vessels in which transformations of substances are carried out and studied, i.e. chemical reactions (Fig. 8).

Rice. 8. Laboratory chemical glassware and equipment

They rightly say that it is better to see once than to hear a hundred times. Or better yet, hold it in your hands and learn how to use it. Therefore, your first acquaintance with chemical equipment will occur during the practical work that awaits you in the next lesson.

1. What is observation? What conditions must be met for observation to be effective?
2. What is the difference between a hypothesis and a conclusion?
3. What is an experiment?
4. What is the structure of a flame?
5. How should heating be carried out?
6. What laboratory equipment did you use when studying biology and geography?
7. What laboratory equipment is used when studying chemistry?

Practical work No. 1.
Familiarization with laboratory equipment.
Safety regulations

Most chemical experiments are carried out in glass containers. The glass is transparent and you can observe what happens to the substances. In some cases, glass is replaced with transparent plastic; it does not break, but such dishes, unlike glass, cannot be heated.

Beakers are often used for demonstration experiments (Fig. 13). Often glasses and conical flasks have special marks, with their help you can approximately determine the volume of liquid contained in them.

Round-bottomed flasks (Fig. 14) cannot be placed on the table; they are secured to metal stands - tripods (Fig. 15) - using claws. The legs, as well as metal rings, are attached to the tripod with special clamps. It is convenient to obtain any substances, such as gases, in round-bottomed flasks. In order to collect the resulting gases, use a flask with an outlet (it is called a Wurtz flask (Fig. 16)) or a test tube with a gas outlet tube.

If the resulting gaseous substances need to be cooled and condensed into liquid, use a glass refrigerator (Fig. 17). Cooled gases move through its inner tube, turning into liquid under the influence of cold water, which flows through the “jacket” of the refrigerator in the opposite direction.

Conical funnels (Fig. 18) are used for pouring liquids from one vessel to another; they are also indispensable in the filtering process. You probably know that filtration is the process of separating liquid from solid particles.

A dish with thick walls, similar to a deep plate, is called a crystallizer (Fig. 20). Due to the large surface area of ​​the solution poured into the crystallizer, the solvent quickly evaporates and the dissolved substance is released in the form of crystals. Under no circumstances should the crystallizer be heated: its walls only seem strong, but in fact, when heated, it will certainly crack.

When performing a chemical experiment, you often have to measure the required volume of liquid. Most often, graduated cylinders are used for this (Fig. 21).

In addition to glassware, the school chemistry laboratory has porcelain dishes. In a mortar and pestle (Fig. 22), crystalline substances are crushed. Glassware is not suitable for this: the pressure of the pestle will cause it to immediately crack.

To avoid troubles and injuries, each item must be used strictly for its intended purpose and know how to handle it. A chemical experiment will be truly safe, instructive and interesting if you take precautions when working with chemical glassware, reagents, and equipment. These measures are called safety regulations.

The chemistry room is an unusual office. This means that the requirements for you here are special. For example, you should never eat in the chemistry lab, since many of the substances you will work with are poisonous.

The chemical room differs from other rooms in that it has a fume hood (Fig. 24). Many substances have a strong, unpleasant odor, and their vapors are not harmless to health. Such substances are handled in a fume hood, from which gaseous substances flow directly into the street.

The bottle with the reagent must be taken so that the label is in the palm of your hand. This is done so that accidental drips do not spoil the inscription.

Some chemicals are toxic, there are reagents that corrode the skin, and many substances are flammable. Special signs on the labels warn about this (Fig. 26, see p. 7).

Do not start an experiment unless you know exactly what and how to do. You must work strictly following the instructions and only with those substances that are necessary for the experiment.

Prepare your workplace, rationally place reagents, dishes, and accessories so that you don’t have to reach across the table, knocking over flasks and test tubes with your sleeve. Do not clutter the table with anything that is not needed for the experiment.

Experiments must be carried out only in clean containers, which means they must be thoroughly washed after work. Wash your hands at the same time.

All manipulations must be carried out above the table.

To determine the smell of a substance, do not bring the vessel close to your face, but push air with your hand from the opening of the vessel to your nose (Fig. 27).

No substances can be tasted!

Never pour excess reagent back into the bottle. Use a special waste glass for this. It is also undesirable to collect spilled solids back, especially with your hands.

If you accidentally burn yourself, cut yourself, or spill reagent on the table, on your hands, or on your clothes, contact your teacher or laboratory assistant immediately.

After finishing the experiment, put your work area in order.

Practical work No. 2.
Watching a burning candle

It would seem that what can be written about such a simple object of observation as a burning candle? However, observation is not only the ability to see, it is the ability to pay attention to details, concentration, the ability to analyze, and sometimes even ordinary perseverance. The great English physicist and chemist M. Faraday wrote: “Consideration of the physical phenomena that occur when a candle burns is the broadest way in which one can approach the study of natural science.”

The purpose of this practical work is to learn to observe and describe the results of observation. You have to write a short miniature essay about a burning candle (Fig. 28). To help you with this, we offer several questions that require detailed answers.

Describe the appearance of the candle, the substance from which it is made (color, smell, feel, hardness), and the wick.

Light a candle. Describe the appearance and structure of the flame. What happens to the candle material when the wick burns? What does the wick look like during the combustion process? Does the candle heat up, is there a sound when burning, is heat released? What happens to a flame if there is air movement?

How quickly does a candle burn out? Does the length of the wick change during the combustion process? What is the liquid at the base of the wick? What happens to it when it is absorbed by the wick material? And when its drops flow down the candle?

Many chemical processes occur when heated, but a candle flame is not used for this purpose. Therefore, in the second part of this practical work, we will get acquainted with the structure and operation of a heating device already familiar to you - an alcohol lamp (Fig. 29). The alcohol lamp consists of a glass tank 1 , which is filled with alcohol to no more than 2/3 of the volume. The wick is immersed in alcohol 2 , which is made from cotton threads. It is held in the neck of the tank using a special tube with a disk 3 . Light the alcohol lamp only with the help of matches; you cannot use another burning alcohol lamp for this purpose, because In this case, spilled alcohol may spill and ignite. The wick must be cut evenly with scissors, otherwise it will begin to burn. To put out an alcohol lamp, do not blow on the flame; a glass cap is used for this purpose. 4 . It also protects the alcohol lamp from rapid evaporation of alcohol.

Used for chemical experiments at school

Let's take a closer look at all types of equipment.

Laboratory glassware, depending on the material from which it consists, it is divided into glass And porcelain .

Glassware based on the presence of special symbols on it, it may be measured And ordinary.

TO glassware relate . We will study all this during practical work.

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3. Techniques for handling laboratory equipment. Watching a burning candle. Flame structure

You already know thatchemical transformations of substances – These are phenomena as a result of which other substances are formed from one substance. They are also called chemical reactions. However, special laboratory equipment is required to carry out chemical reactions.

Used for chemical experiments at schoolspecial laboratory glassware, tripod and heating devices.

Let's take a closer look at all types of equipment.

Laboratory glassware,depending on the material from which it consists, it is divided into glass and porcelain.

Glasswarebased on the presence of special symbols on it, it may be measured and ordinary.

TO glassware include test tubes, flasks, beakers, funnels, pipettes, flasks.

Test tubes – used when conducting experiments for solutions, gases and solids.

Flasks There are flat-bottomed and conical. They are used in the same way as test tubes. Similarly usedbeakers.

Funnels are used for pouring a solution into a vessel with a narrow neck and for filtering liquids and, depending on the structure, are divided intoconical and drip.

Pipettes used to remove a certain volume of liquid from a flask.

TO porcelain dishes include mortar, pestles, Buchner funnel, crucible, glass, spoon, spatula, evaporating bowls.

Mortar and pestles used for grinding substances.

Crucible used for heating and calcining substances.

Glass, spoon, spatula– for pouring dry chemicals into other laboratory glassware.

Evaporation bowlsused for evaporation of various solutions.

Buchner funnel - Designed for filtering under vacuum. The upper part of the funnel, into which the liquid is poured, is separated by a porous or perforated partition from the lower part, to which a vacuum is applied.

Tripod serves to secure laboratory glassware, accessories and instruments when performing experiments. It consists of a stand into which a rod is screwed. The stand gives the tripod stability. A ring, a tab, a clamp and a mesh can be attached to the rod using couplings. The coupling has a screw, when loosened, it is possible to move and secure the ring, tab, clamp and mesh along the rod. Each of the listed holders is used to secure laboratory glassware in it.

TO heating devices include alcohol lamp, gas burner and electric heater.

Alcohol lamp consists of a vessel with alcohol, a wick mounted in a metal tube with a disk, and a cap.

When carrying out laboratory and practical work, it is necessary to observebasic safety rules:

1. Use only substances specified by the teacher in accordance with their intended purpose.
2. Do not clutter your workplace with unnecessary items.
3. Do not start work without precise instructions from the teacher.
4. Check the integrity and cleanliness of laboratory glassware before use.
5. Do not taste chemicals or handle them with your hands (only with a spatula or test tube!). It is prohibited to determine the composition of chemical substances by smell.
6. When heating substances, the test tube should be held away from you. Do not point the opening of the test tube at people.
7. Be sure to close the vessels after taking chemicals from them.

We will carry out practical work on studying the structure of the flame, working with an alcohol lamp.

1. Remove the cap from the spirit lamp and check whether the disc fits tightly to the opening of the vessel.This is necessary to prevent the alcohol from igniting..
2. We light the alcohol lamp with a burning match.It is not allowed to light an alcohol lamp of another burning alcohol lamp to avoid a fire.

When consideringthe structure of the flame itself, we will notice three zones having different temperatures:

1. Lower The (dark) part of the flame is cold. There is no combustion there;
2. Average (brightest), where, under the influence of high temperature, carbon-containing compounds decompose, and coal particles become heated, emitting light;
3. External (lightest), where the most complete combustion of decomposition products occurs with the formation of carbon dioxide and water.

1. To confirm the presence of these zones, we use an ordinary splinter or a thick match. We bring it into the flame horizontally, as if “piercing” all three combustion zones of the alcohol lamp. We examine it after extraction. We notice more and less charred zones, confirming the non-uniformity of temperature in the flame of an alcohol lamp.
2. Extinguishing the flame of an alcohol lamp is done by covering it with a cap.

Conclusion: The flame consists of three zones (lower, middle and outer), the structure of which depends on the chemical composition of the flame.

Chemistry - one of the sciences that helps to understand the secrets of nature.

After all, one of the necessary skills is the ability to distinguish physical phenomena from chemical ones by observing various phenomena in nature.

To more fully understand these phenomena, let us observe the changes that occur with a burning candle. Let's take a paraffin candle and light it.

1. Watching how paraffin melts, we notice that it does not change its properties, but only changes its shape.

From previous lessons we know thatphysical phenomena- these are phenomena as a result of which the size, shape of bodies or the state of aggregation of substances change, but their composition remains constant.

This means that this phenomenon when a candle burns refers to physical phenomena.

1. At the same time, the candle wick, when burned, forms ash.

Let's remember whatchemical phenomenarefer to phenomena as a result of which other substances are formed from one substance.

This means that this phenomenon refers to chemical phenomena.

A burning candle is just one example of the simultaneous presence and interconnection of physical and chemical phenomena in nature. In fact, these phenomena surround us everywhere. And by being observant, we can notice them in everyday life.

Target: learn to describe the results of observations.

Reagents and equipment: paraffin candle, lime water; a splinter, a glass tube with an extended end, a beaker, a graduated cylinder, matches, a porcelain object (a porcelain cup for evaporation), crucible tongs, a test tube holder, glass jars with a volume of 0.5, 0.8, 1, 2, 3, 5 l, a stopwatch.

Task 1. Observing a burning candle.

Present your observations in the form of a short essay. Draw a candle flame.

The candle consists of paraffin and has a specific smell. There is a wick in the middle.
When the wick burns, the candle melts. A slight track is heard and heat is generated.

Task 2. Study of various parts of the flame.

1. The flame, as you already know, has three zones. Which? When examining the lower part of the flame, bring the end of the glass tube into it using crucible tongs, holding it at an angle of 45-50 degrees. Bring a burning splinter to the other end of the tube. What are you observing?

Combustion produces heat.

2. In order to study the middle part of the flame, the brightest, insert a porcelain bowl into it (using crucible tongs) for 2-3 seconds. What did you find?

Blackening.

3. To study the composition of the upper part of the flame, insert an overturned beaker moistened with lime water into it for 2-3 seconds so that the flame is in the middle of the beaker. What are you observing?

Formation of solid sediment.

4. To establish the difference in temperature in different parts of the flame, insert a splinter into the lower part of the flame for 2-3 seconds (so that it crosses all its parts horizontally). What are you observing?

The top part burns faster.

5. Complete the report by filling out table 4.

Task 3. Study of the rate of oxygen consumption during combustion.

1. Light a candle and cover it with a 0.5 liter jar. Determine the time during which the candle burns.

Carry out similar actions using jars of other sizes.

Fill out table 5.

The duration of candle burning depends on the volume of air.

2. Draw a graph of the duration of candle burning versus the volume of the jar (air). Use it to determine the time after which a candle covered with a 10 liter jar will go out.

3. Calculate the time during which the candle will burn in a closed school office.

The length of the school chemistry classroom (a) is 5 m, the width (b) is 5 m, and the height (c) is 3 m.
The volume of a school chemistry classroom is 75 cubic meters. or 75000 l. The time during which the candle will burn, taking into account the fact that no air enters the room and all the oxygen is spent on burning the candle, is 2,700,000 s or 750 hours.

Task 4. Introducing the structure of a spirit lamp.

1. Look at Figure 2 and write the name of each part of the spirit lamp. You will find the necessary information on page 23 of the textbook.

1. Alcohol
2. Wick
3. Wick holder
4. Cap

a) Why is the match held from the side when lighting a spirit lamp?

To avoid getting burned.

b) Why can’t you light a spirit lamp from another burning spirit lamp?

Alcohol may spill and catch fire.

2. Using the equipment at your desk, boil water in a test tube.

The figure shows how much water should be in the test tube, how to properly secure it in the holder or in the tripod leg, and into what part of the flame the test tube should be placed.

a) How much water should be poured into the test tube?

2/3 test tubes.

b) How to hold a test tube over the flame of an alcohol lamp?

At an angle away from you.

Fire itself is a symbol of life; its importance can hardly be overestimated, since since ancient times it has helped a person to keep warm, see in the dark, cook delicious dishes, and also protect himself.

History of the flame

Fire has accompanied man since the primitive times. A fire burned in the cave, insulating and illuminating it, and when going for prey, hunters took burning brands with them. They were replaced by tarred torches - sticks. With their help, the dark and cold castles of the feudal lords were illuminated, and huge fireplaces heated the halls. In ancient times, the Greeks used oil lamps - clay teapots with oil. In the 10th and 11th centuries, wax and tallow candles began to be created.

A torch burned in a Russian hut for many centuries, and when kerosene began to be extracted from oil in the mid-19th century, kerosene lamps came into use, and later gas burners. Scientists are still studying the structure of the flame, discovering new possibilities.

Color and intensity of fire

Oxygen is required to produce a flame. The more oxygen, the better the combustion process. If you fan the heat, then fresh air enters it, which means oxygen, and when smoldering pieces of wood or coals flare up, a flame appears.

Flames come in different colors. The wood fire flames dance in yellow, orange, white and blue colors. The color of the flame depends on two factors: the combustion temperature and the material being burned. In order to see the dependence of color on temperature, it is enough to monitor the heat of an electric stove. Immediately after switching on, the coils heat up and begin to glow dull red.

The more they heat up, the brighter they become. And when the coils reach their highest temperature, they turn a bright orange color. If you could heat them up even more, they would change their color to yellow, white, and eventually blue. The blue color would indicate the highest heat level. The same thing happens with fire.

What does the structure of a flame depend on?

It flickers in different colors as the wick burns through the melting wax. Fire requires access to oxygen. When a candle burns, much oxygen does not get into the middle of the flame, near the bottom. That's why it looks darker. But the top and sides get a lot of air, so the flame is very bright there. It heats up to more than 1370 degrees Celsius, which makes the candle flame mostly yellow in color.

And in the fireplace or in the fire at a picnic you can see even more flowers. A wood fire burns at a temperature lower than a candle. That's why it looks more orange than yellow. Some carbon particles in the fire are very hot and give it a yellow color. Minerals and metals such as calcium, sodium, copper, heated to high temperatures, give the fire a variety of colors.

Flame color

Chemistry in the structure of the flame plays a significant role, because its different shades come from different chemical elements that are in the burning fuel. For example, fire may contain sodium, which is part of the salt. When sodium burns, it emits a bright yellow light. There may also be calcium, a mineral, in the fire. For example, there is a lot of calcium in milk. When calcium is heated, it emits a dark red light. And if a mineral such as phosphorus is present in the fire, it will give a greenish color. All these elements can be in the wood itself or in other materials caught in the fire. Eventually, mixing all these different colors in a flame can form the color white - just like a rainbow of colors brought together to form sunlight.

Where does fire come from?

The flame structure diagram represents gases in a burning state, in which there are composite plasmas or solid dispersed substances. Physical and chemical transformations occur in them, which are accompanied by glow, heat release and heating.

Tongues of flame form processes accompanied by combustion of a substance. Compared to air, gas has a lower density, but under the influence of high temperature it rises. This is how you get long or short flames. Most often, there is a soft flow of one form into another. To see this phenomenon, you can turn on the burner of a regular gas stove.

The fire ignited in this case will not be uniform. Visually, the flame can be divided into three main zones. A simple study of the structure of the flame indicates that different substances burn with the formation of different types of torch.

When the gas-air mixture is ignited, a short flame with a blue and violet tint is first formed. In it you can see a green-blue core in the shape of a triangle.

Flame zones

Considering the structure of the flame, three zones are distinguished: first, preliminary, where heating of the mixture emerging from the burner opening begins. After it comes the zone where the combustion process takes place. This area covers the top of the cone. When there is not enough air flow, gas combustion occurs partially. This produces carbon monoxide and hydrogen residues. Their combustion occurs in the third zone, where there is good access to oxygen.

For example, let’s imagine the structure of a candle flame.

The combustion scheme includes:

• the first is the dark zone;
• the second - the glow zone;
• the third is a transparent zone.

The candle thread does not burn, but only charring of the wick occurs.

The structure of a candle flame is a hot gas flow rising upward. The process begins with heating until the paraffin evaporates. The area adjacent to the thread is called the first area. It has a slight blue glow due to an excess amount of flammable material, but a small supply of oxygen. Here the process of partial combustion of substances occurs with the formation of fumes, which then oxidizes.

The first zone is covered by a luminous shell. It contains a sufficient amount of oxygen, which promotes the oxidative reaction. It is here that, with intense heating of particles of remaining fuel and coal particles, a glow effect is observed.

The second zone is covered by a barely noticeable shell with a high temperature. A lot of oxygen penetrates into it, which promotes complete combustion of fuel particles.

Alcohol lamp flame

For various chemical experiments, small containers with alcohol are used. They are called alcohol lamps. The structure of the flame is similar to a candle flame, but still has its own characteristics. The wick leaks alcohol, which is facilitated by capillary pressure. When the top of the wick is reached, the alcohol evaporates. In the form of steam, it ignites and burns at a temperature of no more than 900 °C.

The structure of the flame of an alcohol lamp has the usual shape, it is almost colorless, with a slightly bluish tint. Its zones are more blurred than those of a candle. In an alcohol burner, the base of the flame is located above the burner grid. The deepening of the flame leads to a decrease in the volume of the dark cone, and a luminous zone emerges from the hole.

Chemical processes in a flame

The oxidation process takes place in an inconspicuous zone, which is located at the top and has the highest temperature. In it, particles of the combustion product undergo final combustion. And excess oxygen and lack of fuel lead to a strong oxidation process. This ability can be used when quickly heating substances over a burner. To do this, the substance is dipped into the top of the flame, where combustion occurs much faster.

Reduction reactions occur in the central and lower parts of the flame. There is a sufficient supply of fuel and a small supply of oxygen necessary for the combustion process. When oxygen-containing substances are added to these zones, oxygen is eliminated.

The process of decomposition of ferrous sulfate is considered as a reducing flame. When FeSO 4 penetrates into the middle of the torch, it first heats up and then decomposes into ferric oxide, anhydride and sulfur dioxide. In this reaction, sulfur is reduced.

Fire temperature

Each area of ​​a candle or burner flame has its own temperature indicators, depending on the access of oxygen. The temperature of the open flame, depending on the zone, can vary from 300 °C to 1600 °C. An example is the diffusion and laminar flame, the structure of its three shells. The flame cone in the dark area has a heating temperature of up to 360 °C. Above it there is a glow zone. Its heating temperature varies from 550 to 850 °C, which leads to the splitting of the combustible mixture and the process of its combustion.

The outer area is slightly noticeable. In it, the heating of the flame reaches 1560 °C, which is explained by the properties of the molecules of the burning substance and the rate of entry of oxidizing agents. Here the combustion process is the most energetic.

Cleansing Fire

The flame contains enormous energy potential; candles are used in rituals of cleansing and forgiveness. And how nice it is to sit near a cozy fireplace on quiet winter evenings, gathering with your family and discussing everything that happened during the day.

Fire and candle flames carry a huge charge of positive energy, because it is not without reason that those sitting by the fireplace feel peace, comfort and tranquility in their souls.

How to curse the darkness
It's better to at least light it
one small candle.
Confucius

At the beginning

The first attempts to understand the combustion mechanism are associated with the names of the Englishman Robert Boyle, the Frenchman Antoine Laurent Lavoisier and the Russian Mikhail Vasilyevich Lomonosov. It turned out that during combustion the substance does not “disappear” anywhere, as was once naively believed, but turns into other substances, mostly gaseous and therefore invisible. Lavoisier was the first to show in 1774 that during combustion, approximately a fifth of it is lost from the air. During the 19th century, scientists studied in detail the physical and chemical processes that accompany combustion. The need for such work was caused primarily by fires and explosions in mines.

But only in the last quarter of the twentieth century were the main chemical reactions accompanying combustion identified, and to this day many dark spots remain in the chemistry of flame. They are studied using the most modern methods in many laboratories. These studies have several goals. On the one hand, it is necessary to optimize combustion processes in the furnaces of thermal power plants and in the cylinders of internal combustion engines, to prevent explosive combustion (detonation) when the air-gasoline mixture is compressed in a car cylinder. On the other hand, it is necessary to reduce the amount of harmful substances formed during the combustion process, and at the same time, to look for more effective means of extinguishing the fire.

There are two types of flame. Fuel and oxidizer (most often oxygen) can be forced or spontaneously supplied to the combustion zone separately and mixed in the flame. Or they can be mixed in advance - such mixtures can burn or even explode in the absence of air, such as gunpowder, pyrotechnic mixtures for fireworks, rocket fuel. Combustion can occur both with the participation of oxygen entering the combustion zone with air, and with the help of oxygen contained in the oxidizing substance. One of these substances is Berthollet salt (potassium chlorate KClO 3); this substance easily gives up oxygen. A strong oxidizing agent is nitric acid HNO 3: in its pure form it ignites many organic substances. Nitrates, salts of nitric acid (for example, in the form of fertilizer - potassium or ammonium nitrate), are highly flammable if mixed with flammable substances. Another powerful oxidizer, nitrogen tetroxide N 2 O 4 is a component of rocket fuels. Oxygen can also be replaced by strong oxidizing agents such as chlorine, in which many substances burn, or fluorine. Pure fluorine is one of the most powerful oxidizing agents; water burns in its stream.

Chain reactions

The foundations of the theory of combustion and flame propagation were laid in the late 20s of the last century. As a result of these studies, branched chain reactions were discovered. For this discovery, Russian physical chemist Nikolai Nikolaevich Semenov and English researcher Cyril Hinshelwood were awarded the Nobel Prize in Chemistry in 1956. Simpler unbranched chain reactions were discovered back in 1913 by the German chemist Max Bodenstein using the example of the reaction of hydrogen with chlorine. The overall reaction is expressed by the simple equation H 2 + Cl 2 = 2HCl. In fact, it involves very active fragments of molecules - the so-called free radicals. Under the influence of light in the ultraviolet and blue regions of the spectrum or at high temperatures, chlorine molecules disintegrate into atoms, which begin a long (sometimes up to a million links) chain of transformations; Each of these transformations is called an elementary reaction:

Cl + H 2 → HCl + H,
H + Cl 2 → HCl + Cl, etc.

At each stage (reaction link), one active center (hydrogen or chlorine atom) disappears and at the same time a new active center appears, continuing the chain. The chains break when two active species meet, for example Cl + Cl → Cl 2. Each chain propagates very quickly, so if the "initial" active particles are generated at high speed, the reaction will proceed so quickly that it can lead to an explosion.

N. N. Semenov and Hinshelwood discovered that the combustion reactions of phosphorus and hydrogen vapors proceed differently: the slightest spark or open flame can cause an explosion even at room temperature. These reactions are branched chain reactions: active particles “multiply” during the reaction, that is, when one active particle disappears, two or three appear. For example, in a mixture of hydrogen and oxygen, which can be quietly stored for hundreds of years if there are no external influences, the appearance of active hydrogen atoms for one reason or another triggers the following process:

H + O 2 → OH + O,
O + H 2 → OH + H.

Thus, in an insignificant period of time, one active particle (H atom) turns into three (a hydrogen atom and two OH hydroxyl radicals), which already launch three chains instead of one. As a result, the number of chains grows like an avalanche, which instantly leads to an explosion of the mixture of hydrogen and oxygen, since a lot of thermal energy is released in this reaction. Oxygen atoms are present in flames and in the combustion of other substances. They can be detected by directing a stream of compressed air across the top of the burner flame. At the same time, a characteristic smell of ozone will be detected in the air - these are oxygen atoms “sticking” to oxygen molecules to form ozone molecules: O + O 2 = O 3, which were carried out of the flame by cold air.

The possibility of an explosion of a mixture of oxygen (or air) with many flammable gases - hydrogen, carbon monoxide, methane, acetylene - depends on the conditions, mainly on the temperature, composition and pressure of the mixture. So, if, as a result of a leak of household gas in the kitchen (it consists mainly of methane), its content in the air exceeds 5%, then the mixture will explode from the flame of a match or lighter, and even from a small spark that slips through the switch when turning on the light. There will be no explosion if the chains break faster than they can branch. This is why the lamp for miners, which the English chemist Humphry Davy developed in 1816, without knowing anything about the chemistry of flame, was safe. In this lamp, the open flame was fenced off from the external atmosphere (which could be explosive) with a thick metal mesh. On the metal surface, active particles effectively disappear, turning into stable molecules, and therefore cannot penetrate into the external environment.

The complete mechanism of branched chain reactions is very complex and can include more than a hundred elementary reactions. Many oxidation and combustion reactions of inorganic and organic compounds are branched chain reactions. The same will be the reaction of fission of nuclei of heavy elements, for example plutonium or uranium, under the influence of neutrons, which act as analogues of active particles in chemical reactions. Penetrating into the nucleus of a heavy element, neutrons cause its fission, which is accompanied by the release of very high energy; At the same time, new neutrons are emitted from the nucleus, which cause the fission of neighboring nuclei. Chemical and nuclear branched chain processes are described by similar mathematical models.

What do you need to get started?

For combustion to begin, a number of conditions must be met. First of all, the temperature of the flammable substance must exceed a certain limit value, which is called the ignition temperature. Ray Bradbury's famous novel Fahrenheit 451 is so named because at approximately this temperature (233°C) paper catches fire. This is the “ignition temperature” above which solid fuels release flammable vapors or gaseous decomposition products in quantities sufficient for their stable combustion. The ignition temperature of dry pine wood is approximately the same.

The flame temperature depends on the nature of the combustible substance and the combustion conditions. Thus, the temperature in a methane flame in air reaches 1900°C, and when burning in oxygen - 2700°C. An even hotter flame is produced by combustion of hydrogen (2800°C) and acetylene (3000°C) in pure oxygen. No wonder the flame of an acetylene torch easily cuts almost any metal. The highest temperature, about 5000°C (it is recorded in the Guinness Book of Records), is obtained when burned in oxygen by a low-boiling liquid - carbon subnitride C 4 N 2 (this substance has the structure of dicyanoacetylene NC–C=C–CN). And according to some information, when it burns in an ozone atmosphere, the temperature can reach up to 5700°C. If this liquid is set on fire in air, it will burn with a red, smoky flame with a green-violet border. On the other hand, cold flames are also known. For example, phosphorus vapors burn at low pressures. A relatively cold flame is also obtained during the oxidation of carbon disulfide and light hydrocarbons under certain conditions; for example, propane produces a cool flame at reduced pressure and temperatures between 260–320°C.

Only in the last quarter of the twentieth century did the mechanism of processes occurring in the flames of many combustible substances begin to become clearer. This mechanism is very complex. The original molecules are usually too large to react directly with oxygen into reaction products. For example, the combustion of octane, one of the components of gasoline, is expressed by the equation 2C 8 H 18 + 25 O 2 = 16 CO 2 + 18 H 2 O. However, all 8 carbon atoms and 18 hydrogen atoms in an octane molecule cannot simultaneously combine with 50 oxygen atoms : for this to happen, many chemical bonds must be broken and many new ones must be formed. The combustion reaction occurs in many stages - so that at each stage only a small number of chemical bonds are broken and formed, and the process consists of many sequentially occurring elementary reactions, the totality of which appears to the observer as a flame. It is difficult to study elementary reactions primarily because the concentrations of reactive intermediate particles in the flame are extremely small.

Inside the flame

Optical probing of different areas of the flame using lasers made it possible to establish the qualitative and quantitative composition of the active particles present there - fragments of molecules of a combustible substance. It turned out that even in the seemingly simple reaction of combustion of hydrogen in oxygen 2H 2 + O 2 = 2H 2 O, more than 20 elementary reactions occur with the participation of molecules O 2, H 2, O 3, H 2 O 2, H 2 O, active particles N, O, OH, BUT 2. Here, for example, is what the English chemist Kenneth Bailey wrote about this reaction in 1937: “The equation for the reaction of hydrogen with oxygen is the first equation that most beginners in chemistry become familiar with. This reaction seems very simple to them. But even professional chemists are somewhat amazed to see a hundred-page book entitled “The Reaction of Oxygen with Hydrogen,” published by Hinshelwood and Williamson in 1934.” To this we can add that in 1948 a much larger monograph by A. B. Nalbandyan and V. V. Voevodsky was published entitled “The Mechanism of Hydrogen Oxidation and Combustion.”

Modern research methods have made it possible to study the individual stages of such processes and measure the rate at which various active particles react with each other and with stable molecules at different temperatures. Knowing the mechanism of individual stages of the process, it is possible to “assemble” the entire process, that is, to simulate a flame. The complexity of such modeling lies not only in studying the entire complex of elementary chemical reactions, but also in the need to take into account the processes of particle diffusion, heat transfer and convection flows in the flame (it is the latter that create the fascinating play of tongues of a burning fire).

Where does everything come from?

The main fuel of modern industry is hydrocarbons, ranging from the simplest, methane, to heavy hydrocarbons, which are contained in fuel oil. The flame of even the simplest hydrocarbon, methane, can involve up to a hundred elementary reactions. However, not all of them have been studied in sufficient detail. When heavy hydrocarbons, such as those found in paraffin, burn, their molecules cannot reach the combustion zone without remaining intact. Even on approaching the flame, due to the high temperature, they split into fragments. In this case, groups containing two carbon atoms are usually split off from molecules, for example C 8 H 18 → C 2 H 5 + C 6 H 13. Active species with an odd number of carbon atoms can abstract hydrogen atoms, forming compounds with double C=C and triple C≡C bonds. It was discovered that in a flame such compounds can enter into reactions that were not previously known to chemists, since they do not occur outside the flame, for example C 2 H 2 + O → CH 2 + CO, CH 2 + O 2 → CO 2 + H + N.

The gradual loss of hydrogen by the initial molecules leads to an increase in the proportion of carbon in them, until particles C 2 H 2, C 2 H, C 2 are formed. The blue-blue flame zone is due to the glow of excited C 2 and CH particles in this zone. If the access of oxygen to the combustion zone is limited, then these particles do not oxidize, but are collected into aggregates - they polymerize according to the scheme C 2 H + C 2 H 2 → C 4 H 2 + H, C 2 H + C 4 H 2 → C 6 H 2 + N, etc.

The result is soot particles consisting almost exclusively of carbon atoms. They are shaped like tiny balls with a diameter of up to 0.1 micrometers, which contain approximately a million carbon atoms. Such particles at high temperatures give a well-luminous yellow flame. At the top of the candle flame, these particles burn, so the candle does not smoke. If further adhesion of these aerosol particles occurs, larger soot particles are formed. As a result, the flame (for example, burning rubber) produces black smoke. Such smoke appears if the proportion of carbon relative to hydrogen in the original fuel is increased. An example is turpentine - a mixture of hydrocarbons with the composition C 10 H 16 (C n H 2n–4), benzene C 6 H 6 (C n H 2n–6), and other flammable liquids with a lack of hydrogen - all of them smoke when burned. A smoky and brightly luminous flame is produced by acetylene C 2 H 2 (C n H 2n–2) burning in air; Once upon a time, such a flame was used in acetylene lanterns mounted on bicycles and cars, and in miners' lamps. And vice versa: hydrocarbons with a high hydrogen content - methane CH 4, ethane C 2 H 6, propane C 3 H 8, butane C 4 H 10 (general formula C n H 2n + 2) - burn with sufficient air access with an almost colorless flame. A mixture of propane and butane in the form of a liquid under low pressure is found in lighters, as well as in cylinders used by summer residents and tourists; the same cylinders are installed in gas-powered cars. More recently, it was discovered that soot often contains spherical molecules consisting of 60 carbon atoms; they were called fullerenes, and the discovery of this new form of carbon was marked by the award of the Nobel Prize in Chemistry in 1996.